Satellite navigation systems provide autonomous geo-spatial positioning with global or regional coverage. At the present, the dominating GNSS is the Global Positioning System (GPS). A GPS receiver has the ability to determine its geographic location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a line-of-sight by radio from satellites.
However, there are situations when it is not possible or even permitted for a portable electronic device with a GPS receiver to receive the satellite signals used for positioning. For example, satellite signals might be obscured or blocked when the handheld device in operated indoors. Still further, the GPS system might have inadequate coverage in a specific geographic area, or the satellite signals may be actively jammed to prevent positioning.
Furthermore, it may be desirable to have the option of providing positioning functionality to a portable electronic device without the need to incorporate a GPS receiver, which adds cost, space and energy consumption to the portable electronic device.
It is well-known that antenna arrays with several physical antenna elements (denoted “physical antenna arrays” in the following) can be used for directional estimation of incoming signals. A portable electronic device with a physical antenna array is e.g. disclosed in US2008/0100502. The portable electronic device is moved from a first to a second position, while estimating the direction of arrival of incoming radio signals at the first and second positions by processing the incoming radio signals received from a signal source by the plural antenna elements at the first and the second position, respectively. Further, a built-in motion detector indicates the displacement vector between the first and second positions. The displacement vector in combination with the directions allows the portable electronic device to be positioned relative to the signal source.
One problem with physical antenna arrays is that they are large and bulky and usually consume more space than a portable electric device can afford. They may also require precise calibration, so that the response of each antenna element is known in all possible directions, in order to enable directional estimation.
In the field of antennas, there are also so-called virtual or synthetic antenna arrays which make use of robots to move a single physical antenna element to a number of known positions. The signals recorded at the different positions can be processed just as data from physical antenna arrays, as long as the surroundings of the antenna are sufficiently static during the measurement, and can therefore also be used for directional estimation. Like physical antenna arrays, virtual antenna arrays are bulky, mainly due to the need for a positioning device (usually some kind of robot or rail). Virtual antenna arrays are generally not developed with size constraints in mind, but are rather used to avoid the requirement for (the often cumbersome) calibration or to avoid coupling effects that may arise between the plural antenna elements of a physical antenna array, see e.g. L. M. Correia: Mobile broadband multimedia networks, Academic press (2006) chapter 6.6. Virtual antenna arrays of this type are thus unsuitable for use in portable electronic devices.
The prior art also comprises an article by Broumandan et al: “Direction of arrival estimation of GNSS signals based on synthetic antenna arrays”, ION GNSS 2007, 25-28 Sep. 2007, pages 1-11. Broumandan discloses a technique for enhancing GNSS accuracy in urban environments, to reduce the influence of interfering signals generated by reflections of the incoming signals on buildings and other scattering objects in urban environments. This is achieved by determining the directions of the interfering signals and applying adaptive antenna algorithms to design a beamformer to place nulls in the directions of the interfering signals, thereby improving the signal quality of the GNSS signals used for global positioning. Broumandan proposes that an antenna array is synthesized by moving a handheld device with a single antenna in an arbitrary direction while continuously sampling the interference signal. The trajectory of the single array is determined by an inertial measurement unit (IMU) in the handheld device. The resulting set of spatial samples together with the trajectory form a synthetic antenna array, which can be processed for determining the direction of arrival for each interfering signal.
The prior art further comprises DE102006037247, which focuses on solving a multi-path problem in connection with time-of-arrival (TOA) or roundtrip-time-of-flight (TOF) positioning techniques, including GPS. The TOA and TOF techniques are based on obtaining measurement signals that represent the amplitude and phase of a transferred signal dependent on the transit time between a mobile station and each of a plurality of stationary stations. The measurement signals are used for calculating the distance to the each stationary station based on the transit times in the same way as for conventional radar systems, see e.g. Merrill Ivan Skolnik: Introduction to Radar Systems, McGraw-Hill (2002), Chapter 1.1. The multi-path problem arises when signal reflections generate further signal paths in addition to the direct signal transmission path between the mobile station and the stationary station. DE102006037247 suggests solving this problem by generating a synthetic aperture which is designed to form a directionally exact antenna, so as to increase signal-to-noise and reduce the influence of signal reflections on the transit time estimates. It is well known that the resolution of an estimated transmit time is inversely proportional to the bandwidth of the signal; therefore the positioning in DE102006037247 requires a broadband radio signal to get adequate estimates of transit time. Furthermore, the positioning in DE102006037247 requires synchronization across all the stationary stations, or synchronization between the mobile station and each of the stationary stations.
Another type of single antenna direction-finding system is known from U.S. Pat. No. 5,502,450. Here, a single antenna is arranged on an aircraft to receive a signal from a source while the aircraft moves along a linear flight path. A system connected to the antenna detects periodically occurring symbols in the signal at two positions along the flight path and calculates, based on the corresponding signal transmit time, the distance to the source at each position. The distance between the positions along the flight path is determined using existing navigational means. Based on these distances and applying trigonometry calculations, the system is able to estimate the angle or the distance to the source at a downstream position along the linear flight path.